The present invention relates to ultrasonic inspection and particularly to an adjustable acoustic mirror for improving ultrasonic inspection through curved surfaces.
When an ultrasonic inspection is performed, a transducer is calibrated on a flat-top block made from the same material as that being inspected, and containing flat bottomed holes of known diameter and known depth from the surface. A set of inspection parameters, such as energy level, operating frequency and water-path, are set and calibrated to a flat-top block calibration standard. The inspection parameters are used to inspect production hardware. In many cases, the same block inspection parameters are used to inspect through curved entry surfaces. Conventional procedure provides that certain curved surface parts with entry surface curvature larger than about 38 cm radius are inspected just like flat-top parts. For radii less than 38 cm, typically the operator will increase the gain (energy level). This compensates for losses due to the curved entry surface. Increasing the gain, however, also increases both the system noise (electronic noise) and the material noise, and for a large number of aircraft engine materials the parts become "uninspectable" (the part fails inspection because of "high noise", i.e. "noise rejects"). The problem is escalated when trying to focus a sound beam below the curved entry surface (subsurface focusing), and, is more pronounced when going through a concave surface than through a convex surface.
Passing a sound beam through a curved surface decreases the effective sensitivity. A concave surface will cause the beam to focus much shorter than the operator expects. A convex surface will defocus the beam, yielding a less sensitive sound beam than the operator expects and that may not ever focus. The severity of each case is dependent on the radius of curvature of each, the smaller (or tighter) the radius the greater the effect on the sound beam. Also, for surfaces with curvature in just one direction, such as a bore or hole for example, the sound beam will not focus since the surface is not symmetric about the center of the transducer beam. The effect of a curved surface on inspection sensitivity is very complex. It is therefore difficult to compensate for the effect of curved surfaces without some form of correction. Such a correction would keep the same sensitivities and beam properties as those of the flat entry surface. Currently, curved parts receive additional inspection gain to compensate for the energy loss at the material boundary. The inspection gain is determined for each radius and inspection depth combination.
Improving ultrasonic inspection capabilities through curved surfaces has been an insurmountable obstacle for many years. Curved surfaces redirect the sound beam, often in an undesirable direction, resulting in loss of energy and resolution. The severity of the incorrect focusing and energy loss is dependent on the magnitude of the surface curvature. The more curvature there is, the more incorrect focusing, energy loss, and resolution loss there will be. Hence, even providing a fixed curvature acoustic mirror for inspection will not be wholly accurate for all curvatures.
An adjustable or precisely interchangeable device is desired, capable of accurately inspecting any curved surface, regardless of its curvature.